Invisible Waves Shape Continental Slopes

Off a coast line, the ocean depth increases very gradually for one hundred miles or so. Beyond this continental shelf, the slope of the ocean floor abruptly increases downward, forming a region called the continental slope, which extends down to the oceanic crust. Physicists at the University of Texas at Austin simulated oceanic conditions by generating waves in a tank containing a model of the ocean floor. Varying the conditions allowed them to gain insight into how a slope of a few degrees is generated on continental shores.

The density of ocean water increases with depth. Under the influence of gravity, layers of ocean water possessing different densities result in invisible waves below the surface, called internal waves. While waves on the surface of the ocean have been extensively studied, the role of the invisible internal waves has been much less studied, particularly in relation to the continental slope.

By measuring the speed and direction of the water and simulating tidal flow, the researchers showed that intense internal waves are generated by tides moving over the ocean floor, if the floor slopes at the particular angle at which internal waves travel. In other words, the experiment provides a new explanation for how continental slope angles are created: intense internal waves that sculpt the seashore are generated only where the angle of the continental slope matches that of internal waves. - NR

The Really Big Chill

A. Vinante et al.Physical Review A (forthcoming)

Physicists in Italy have set a new cooling record by lowering the temperature of a one ton bar of aluminum alloy to only 0.17 thousandths of a degree Kelvin (170 millionths of a degree above absolute zero). Such a low temperature is a significant step in the quest to create a large object that plays by the same quantum mechanical rules that govern atoms and other tiny objects.

Last year, physicists working on ways stabilize mirrors for the Laser Interferometer Gravitational-wave Observatory (LIGO) chilled a dime-sized mirror to 6.9 thousandths of a degree. The Italian research team smashed the LIGO record both in temperature and mass, in part because they have a better motion sensor that is capable of measuring vibrations of the massive oscillator down to the order of hundredths of an attometer in a second. That's roughly equivalent to detecting a change in the distance from the sun to Neptune with a precision smaller than the diameter of a typical bacterium.

The issue of whether or not objects larger than atoms could exhibit quantum mechanical behavior (such as tunneling through barriers or becoming entangled with other quantum mechanical objects) has been argued ever since the discovery of quantum mechanics. One thing is clear -- for there to be any possibility of quantum behavior in a large object it has to be very cold.

The authors hope to approach the quantum threshold by cooling a massive object down to about one millionth of a degree. While you shouldn't expect the quantum behavior of a super-cold object to include tunneling through walls to adjacent labs, the quantumness of the massive object should become apparent in experiments when it interacts with more common quantum things like photons.

Ironically, the one ton bar they chilled is part of the AURIGA resonant bar gravitational wave detector, which was a one-time competitor to LIGO in the race to detect gravitational waves. – JR

50 Years of PRL

Martin Blume

Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of visit Marty's Milestone PRL project.

This week, Marty is taking a look at milestone papers that led to the Free Electron Laser.

The theory of the free electron laser (FEL), in which a beam of electrons from an accelerator is passed through a periodic transverse magnetic field, was given by Madey in a 1971 paper [J. M. J. Madey, J. Appl. Phys. 42, 1906 (1971)]. The experimental demonstration of the laser was done in two stages and was presented in the above Letters. (While the second Letter appeared in 1977 it is combined here as part of the 1976 Milestones selection.) The first of these Letters shows the gain in the intensity of radiation from a CO2 laser as it passed along the electron beam in the periodic magnetic field. In work reported in the second Letter the incident radiation was replaced by a mirrored cavity, so that the electromagnetic radiation made multiple passes through the magnetic field and through the electron bunches. The lasing action produced radiation with a narrow spectral line and high peak power.

Many free electron lasers have been built around the world, with operation at shorter and shorter wavelengths. At present there are projects under construction to make free electron lasers operating at X-ray wavelengths. Many applications of FELs have been both demonstrated and proposed. See a US National Academies 1994 report (www.nap.edu/openbook.php?isbn=NI000099), and a world wide web virtual library report (http://sbfel3.ucsb.edu/www/vl_fel.html) for further information.

Nadia Ramlagan and James Riordon contributed to this Tip Sheet.

About APS

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